1. Field of the Invention
The present invention relates to a piston burette for a burette apparatus with a burette cylinder that has an outlet opening at one end and in which a piston is slidably mounted.
2. Prior Art
A burette is a transparent tube or cylinder that can be filled with a reagent and that is provided with an etched scale and an outlet opening for controlled delivery of the reagent. The burette is used in quantitative analysis for volumetric measurement of materials. The piston burette is a special form of a burette that is used primarily in analytical chemistry for determination of unknown amounts of materials in samples by means of a titration or in microbiology or genetic engineering. They require a highly precise working device, so that the geometric structure of the piston burette has high precision specifications, especially for the inner diameter of the burette cylinder.
While in the classical burette the reagent is dispensed by means of gravity from a vertically standing glass tube filled with reagent and provided with a tap or stopcock, in the piston burette the burette cylinder is formed as a glass tube in which a piston is slidably mounted to supply or dispense the reagent. The metered volume is measured and indicated by displacement of the piston.
Piston burettes of this type are part of the current state of the art.
FIG. 6 shows the principle of operation of the prior art piston burette. A piston 3 that can move up and down manually or by means of an unshown motor is arranged in a calibrated glass tube acting as the burette cylinder. Two valves 1,2 are required for operation of the known piston burette. The suction valve 1 opens during withdrawal of the reagent from the cylinder 4 and the outlet valve 2 opens when the reagent is forced out from the cylinder. In the idle state of the piston--and of course in every arbitrary position between the upper and lower end points acting as limits of its travel--both valves are closed. The position of the valves is shown in FIG. 6.
In analytical chemistry applications the suction valve 1 is connected by a suction tube 6 with a supply container 7 in which a reagent of known composition and concentration is located. The outlet valve 2 is connected by means of a delivery tube 5 with an apparatus, usually a titration tip, which is located above an unshown receiving flask containing an unknown solution (analysis flask).
During the analysis the reagent is drawn from the supply vessel 7 by a downward motion of the piston 3 and, after switching the valve system, delivered into the analysis flask in exact volume increments via the titration tip by means of piston motion in the other direction. The analysis is ended, when the reaction in the analysis flask is completed which is established with the help of a sensor or a color change occurring in the analysis flask. The amount of reagent solution delivered is read from the scale of the piston burette and determined from the number of the volume increments and the analysis results calculated.
The piston burette is integrated in a burette apparatus including other components. It requires a comparatively expensive valve system connected via two openings with the burette cylinder, since the accuracy of the volume determination depends on the precision with which the valve system operates, especially regarding the possible leakage and total volume of the valve system. Furthermore a highly constant inner diameter over the entire length of the burette cylinder is decisive for the accuracy and precision of the volume determination. For that reason a special calibrated glass tube, the so-called precision dilution device, that is shrunk on an inner mandrel in a hot molding process, is usually used. The known burette cylinder made from glass is thus very expensive and has the disadvantage that it is susceptible to breakage. Multiple usage of the known burette cylinder is somewhat difficult because of the necessity of troublesome cleaning work when the reagent is changed. The valve system, the piston burette the vacuum or suction tubing from the reaction vessel and the piston must be cleaned, dried and reassembled which is a disadvantage.
Furthermore a separate supply vessel is required for the reagents that is connected by means of connecting tubes with the valve system.
A metering device with a piston/cylinder system that is not a piston burette, but which of course can be used in an analogous manner as a burette, is known from German Patent Application DE 43 10 808 A1. This known metering device, which can be formed as a pre-filled device in the same sense as a one-time-use product, is based on glass cylinders made in the usual manner that have diameters that fluctuate from cylinder to cylinder and accounts for these diameter fluctuations, because the interior diameter is measured for each individual burette cylinder and supplied preferably in the form of a machine readable bar code, i.e. marked on it in the case of the bar code and is used in electronic computations of the amount of material metered from the device.
This known device has decisive disadvantages.
The individual inner diameter must be measured for each individual metering cylinder and supplied or marked individually in an expensive process.
Furthermore with pre-filled metering cylinders a change of the metering cylinders by electronic diameter preparation' is difficult. The new diameters must be set in the electronic metering unit in operation and that requires trained operating personnel.
An additional disadvantage of this known metering cylinder is that diameter-adjusted piston plugs having different diameters must be used, which makes a search process' necessary in the manufacture of pre-filled metering cylinders and also suitable storage and logistics are required. If this is not done, i.e. if one does not operate with a plug of a predetermined diameter, mechanical problems result because the metering cylinder is too narrow which in the worst case can lead to a friction force between the piston and cylinder wall which cannot be overcome, the above-named reference not mentioning any type of lubricant in any embodiment.